For example, ink for printing and a coating material have been manufactured by using a mill base in which powder pigment is dispersed in a varnish or a solvent at a high concentration. It is preferable that a process in which powder pigment is dispersed in a solvent or the like be performed such that powder pigment of secondary particles in a state where primary particles of the pigment have been aggregated are crushed and dispersed in a solvent to form fine pigment particles in which coarse particles do not exist in order to improve the coloring power of the ink for printing or the coating material.
Hitherto, as the dispersing apparatus, a sand mill, a grain mill, a ball mill, an attritor and the like have been known. Among the dispersing apparatuses above, a structure for continuously performing the dispersing process and arranged as shown in FIG. 7 has been known.
That is, the structure is a horizontal structure having a cylindrical vessel 101 disposed horizontally. In the vessel 101, a rotational shaft 103 is horizontally and rotatively disposed. A plurality of pin type stirring blades 105 projecting in the radial directions are provided for the rotational shaft 103 to be disposed apart from one another at arbitrary intervals in the axial direction. In the vessel 101, spherical particle media 107 made of, for example, steel, ceramics or stones, are enclosed in order to perform the process for dispersing the material.
With the foregoing structure, when the rotational shaft 103 is rotated by a motor or the like and a raw material for a mill base is supplied through a supply port 109 formed at an end of the vessel 101, the particle media 107 are stirred by the plurality of stirring blades 105 provided for the rotational shaft 103. Therefore, the process for dispersing the raw material for the mill base can be performed. The mill base, subjected to the dispersing process, is continuously discharged through a discharge port 111 formed at another end of the vessel 101.
The foregoing structure sometimes encounters a so-called short pass in which the raw material for the mill base supplied into the vessel 101 through the supply port 109 cannot uniformly be dispersed and therefore the mill base containing coarse pigment particles is discharged through the discharge port 111. Therefore, there arises a problem in that the dispersing process cannot satisfactorily be performed.
When the movement of the particle media 107 is observed, the particle media 107 are in a tendency to follow the rotation of the stirring blades 105 provided for the rotational shaft 103 and rotate together with the same. Therefore, there arises a problem in that the dispersing process cannot effectively be performed.
If the rate of charging the particle media 107 into the vessel 101 is raised in order to prevent the short pass, the short pass can somewhat be prevented. If the rate of charging the particle media 107 Is raised excessively, a choking phenomenon takes place in which the particle media 107 are, in the J101, moved eccentrically toward the discharge port 111. Thus, another problem arises in that the operation cannot be performed safely. Accordingly, the rate of charging of the particle media is generally determined to be 75 to 80% at the time of performing the operation.
A conventional structure shown in FIG. 8 can be available. The structure is a vertical structure in which a cylindrical vessel 101 is disposed vertically. A rotational shaft 103 having stirring blades 105 is vertically and rotatively disposed.
The foregoing structure is formed by converting the horizontal structure into a vertical structure in which the raw material for the mill base is supplied into the vessel 101 through a supply port 109 opened and formed in the upper portion of the vessel 101. Moreover, the rotational shaft 103 is rotated to stir the particle media 107 so that the process for dispersing the raw material for the mill base is performed. The mill base subjected to the dispersing process is discharged through the discharge port 111 formed in the lower portion of the vessel 101. The discharge port 111 has a particle-media separation mechanism 113 in the form of, for example, a lattice or a net and arranged to prevent discharge of the particle media 107 and a raw-material discharge valve 115 capable of opening/closing the discharge port 111.
Since the foregoing structure is formed by simply converting the vessel 101 from the horizontal structure into the vertical structure, a problem similar to that suffered with the horizontal structure arises.
Another conventional structure is arranged as shown in FIGS. 9 and 10. Schematically, the foregoing structure is arranged such that first and second rotational shafts 117A and 117B are vertically disposed in a vertical and cylindrical vessel 101. Plate-like first and second stirring blades 119A and 119B having phases shifted from each other by 90.degree. are provided for the first and second rotational shafts 117A and 117B so as to perform rotation while preventing interference of the first and second stirring blades 119A and 119B.
With the foregoing structure, portions of the loci of rotations of the first and second stirring blades 119A and 119B overlap. However, since each of the first and second stirring blades 119A and 119B has a plate-like shape, a portion of the raw material for the mill base is rotated together in the vessel 101. Moreover, portions adjacent to regions 121A and 121B are outside the rotational regions for the first and second stirring blades 119A and 119B. Thus, there arises a problem in that the process for dispersing the raw material for the mill base cannot satisfactorily be performed and the same is made to be non-uniform.
As prior arts considered to be related to the present invention, there are inventions disclosed in Japanese Patent Laid-Open No. 1-224057 (Prior Art 1), U.S. Pat. No. 4,673,134 (Prior Art 2), U.S. Pat. No. 3,199,792 (Prior Art 3), U.S. Pat No. 4,919,347 (Prior Art 4), and U.S. Pat. No. 4,998,678 (Prior Art 5).
The Prior Art 1 has a structure such that first and second rotational shafts are vertically and rotatively disposed in a vessel having an oblong cross sectional shape; and portions of rotation loci of the first and second stirring blades provided for the first and second rotational shafts overlap. However, dead spaces each having a substantially a triangular shape surrounded by the inner surface of the vessel and the rotational loci are formed in front and rear of the portion in which the loci of rotations of the first and second stirring blades overlap and on the two sides of the same when viewed in the rotational direction of the first and second stirring blades. The raw material for the mill base located in the dead spaces cannot satisfactorily be dispersed and the same can easily be made non-uniform.
The Prior Art 2 has disclosed a structure such that stirring blades are provided for a plurality of rotational shafts. Also the structure of the Prior Art 2 encounters the generation of the substantially triangular dead space between the rotation loci of the stirring blades and the inner surface of the vessel. Thus, a problem similar to that experience with the Prior Art 1 arises.
FIG. 8 of Prior Art 3 discloses a structure in which first and second rotational shafts are vertically and rotatively disposed in a vessel having a shape formed by combining two circular arc curved planes; and stirring blades extending in three directions are provided for the first and second rotational shafts. Each of the three stirring blades has a plate-like shape and arranged to be orated in opposite directions. Moreover, their rotation loci are in contact with each other. Although the problem of the dead space can therefore be overcome, the particle media and the like are in a tendency of easily rotating together with the stirring blades. Thus, there arises a problem in that the raw material for the mill base cannot satisfactorily be dispersed.
The Prior Art 4 has disclosed a structure in which cylindrical first and second rotors each having a multiplicity of projections and pits on the outer surfaces thereof are disposed in a vessel having a shape formed by combining two circular arc curved planes. The foregoing structure has a problem in that the outer surface of the first rotor is not engaged with the outer surface of the second rotor, therefore the rotation loci of the rotors do not overlap, and that the process for manufacturing the rotor becomes too complicated.
The Prior Art 5 has disclosed a structure such that a rotational shaft is vertically and rotatively disposed at an eccentric position in a rotative, vertical and cylindrical vessel. Moreover, a plurality of discs having a plurality of holes in the vicinity of the outer ends thereof are provided for the rotational shaft. Since the foregoing structure is arranged such that the vessel is rotated and the rotational shaft disposed at an eccentric position in the vessel is rotated, there arises a problem in that the overall structure becomes too complicated.